Crush resistant adsorptive agglomerates of activated alumina

ABSTRACT

Activated alumina agglomerates having a crush resistance of at least 12 daN, measured on that fraction thereof having a mean diameter ranging from 3 to 4 mm, and an adsorptive capacity greater than 18% by weight, well adopted as adsorbents in a wide variety of purification processes, are prepared by agglomerating activated alumina particulates, maturing the resulting agglomerates in the presence of a complexing agent for the cation Al 3+ , and then thermally treating the matured agglomerates at a temperature ranging from about 100° to 500° C.

This application is a continuation of application No. 08/370,219, filedJan. 9, 1995, abandoned, a continuation of Ser. No. 07/932,945, filedSep. 15, 1992, abandoned, a continuation of Ser. No. 07/818,904, filedJan. 10, 1992, U.S. Pat. No. 5,210,063, and a continuation of Ser. No.07/659,881, filed on Feb. 25, 1991, abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to novel activated alumina agglomerateshaving good properties of adsorption and improved mechanical propertiesand to a process for the production thereof.

This invention especially relates to small spheres of such activatedalumina.

2. Description of the Prior Art

The use of alumina as an adsorbent for eliminating impurities present ina fluid, which can be liquid or gaseous, is known to this art. Thealumina is used particularly to remove water contained in a current ofgas.

Generally, the above purification is carried out by passing the fluidthrough a bed of alumina agglomerates, preferably in the form of smallspheres stacked inside a purification column.

Consequently, alumina agglomerates destined for use as adsorbents mustnot only have a good adsorptive capacity, but also excellent mechanicalproperties, in particular a good resistance of crushing (crushstrength).

Heretofore, however, it has been found that, as the mechanicalproperties of alumina agglomerates are increased, their adsorptionproperties decrease.

SUMMARY OF THE INVENTION

Accordingly, a major object of the present invention is the provision ofnovel alumina agglomerates having both good adsorption capacity andimproved mechanical properties.

Another object of the present invention is the provision of such novelalumina agglomerates in the form of small spheres that can be easilytransported for charging into and discharging from purification columns,for example by pneumatic systems.

Still another object of this invention is the provision of particularprocess for the production of such novel alumina agglomerates and smallspheres comprised thereof.

Briefly, the present invention features agglomerates of activatedalumina having a crush resistance greater than 12 daN, measured onagglomerates of activated alumina having a mean diameter ranging from 3to 4 mm, and an adsorption capacity greater than 18%, expressed aspercentage by weight of water adsorbed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electron photomicrograph showing the surface morphology ofalumina agglomerates prepared using a complexing agent according to thepresent invention;

FIG. 2 is an electron photomicrograph showing the surface morphology ofalumina agglomerates prepared in the absence of any complexing agent;and

FIG. 3 is a graph plotting crush strength versus particle size ofalumina agglomerates prepared using a complexing agent according to theinvention and of those agglomerates prepared in the absence of anycomplexing agent.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

More particularly according to the present invention, by "mean diameter"is intended a diameter such that 50% by weight of the agglomerates havea diameter greater or less than the mean diameter.

The agglomerates of activated alumina of the invention are preferably inthe form of small spheres having a mean diameter ranging from about 1 to10 mm. They have a resistance to crushing greater than 12 daN,preferably ranging from 12 to 25 daN. In a preferred embodiment of theinvention such resistance is greater than 15 daN, and most preferablyranges from 15 to 20 daN.

It will be appreciated that the values of crush resistance given aboveare with respect to alumina agglomerates having a mean diameter rangingfrom 3 to 4 mm. Such values are thus defined with reference to the meandiameter of the agglomerates.

Therefore, activated alumina agglomerates according to the inventionhave a mean diameter ranging from 6 to 7 mm and display a crushresistance greater than 22 daN, preferably greater than 25 daN and morepreferably ranging from 30 to 50 daN.

The other characterizing property of the activated alumina agglomeratesof the invention is a good capacity for adsorption of water, greaterthan 18% and preferably ranging from 20% to 24%.

In comparison with the small spheres of alumina conventionally used asadsorbents, the alumina agglomerates of the invention have thecharacteristic of possessing a greater crush resistance for equivalentadsorptive properties.

By way of comparison with small spheres of alumina commerciallyavailable form Rhone-Poulenc under the Trademark SPHERALITE 501.4, thesehave a good capacity of adsorption of 18% to 24%, but a resistance tocrushing of only 10 daN, which would be desirable to increase.

In a preferred embodiment of the invention, the activated aluminaagglomerates are also distinguished by a surface morphology includingsmall rods, i.e., in the form of small cylindrical rods. This morphologycan be correlated with that of agglomerates obtained before calcinationand, more precisely, following the rehydration of the activated alumina.As utilized hereinafter, the morphology of said agglomerates will bedesignated as that of "intermediate alumina agglomerates". It ispossible to delineate other structural, textural and mechanicalcharacteristics of the agglomerates of the invention.

The process for preparing the activated alumina agglomerates of theinvention comprises maturing (aging) activated alumina agglomeratesunder atmospheric pressure in the presence of a complexing agent for theAl³⁺ cation, and then thermally treating said agglomerates at atemperature ranging from 100° to about 500° C.

It has now been determined that the solidity of the activated aluminaagglomerates can be increased by influencing the rehydration ofactivated alumina towards the formation of boehmite, by adding aneffective amount of a complexing agent prior to the maturing operation.

In a conventional procedure for making small spheres of activatedalumina, activated alumina powder is ground and granulated, thenmatured. During these two steps, the activated alumina powder isrehydrated to boehmite (Al₂ O₃.H₂ O) and bayerite (Al₂ O₃.3H₂ O). Uponcompletion of the maturing, the rehydration rate of the alumina is atleast equal to 30% and generally favors the formation of up to 20% to25% of bayerite and less than 5% of boehmite.

In accordance with the process of the invention, the alumina isrehydrated into an amount of boehmite of at least 5%, preferably 8% to20%; thus, bayerite constitutes the complement at least equal to 30%.Accordingly, it has now also unexpectedly been found that the increasein the amount of boehmite enables the mechanical properties of the finalagglomerates to be improved without very much affecting theiradsorption.

A first embodiment of the process of the invention entails introducingthe complexing agent during the operation of grinding the activatedalumina powder.

A second embodiment of the process of the invention entails introducingthe complexing agent during the operation of agglomerating the activatedalumina powder.

A third embodiment of carrying out the invention entails optionallygrinding the activated alumina powder, then agglomerating it andtreating the resulting agglomerates with the complexing agent, beforeeffecting the maturing thereof.

The activated alumina agglomerates employed according to the presentinvention are prepared starting from an activated alumina powder havinga poorly crystallized and/or amorphous structure.

In such process by "alumina of poorly crystalline structure" is intendedan alumina such that X-ray analysis gives a diagram that presents onlyone or a few diffuse lines corresponding to crystalline phases oflow-temperature transition aluminas and essentially containing thephases chi, rho, eta, gamma and pseudo-gamma, and mixtures thereof.

By "alumina of amorphous structure" is intended an alumina such thatX-ray analysis thereof indicates no line whatsoever characteristic of acrystalline phase.

The activated alumina used in the invention is generally provided byrapid dehydration of aluminum hydroxides such as bayerite, hydrargilliteor gibbsite, norstrandite or aluminum oxyhydroxides such as boehmite anddiaspore.

This dehydration is effected by using a current of hot gases whichpermits elimination and very rapid entrainment of the water evaporated.The temperature of the gases in the apparatus generally ranges fromabout 400° to 1200° C. with a contact time of the hydroxide with the hotgases on the order of a fraction of a second to four or five seconds.

The alumina thus prepared can be used as such, or, depending on theapplication envisaged, can be treated to eliminate, in particular, thealkalis present: a content of Na₂ O less than 0.5% will be preferred.

The specific surface, measured by the BET method of the activatedalumina prepared by rapid dehydration of hydroxides or oxyhydroxides ofaluminum, generally ranges from about 50 to 400 m² /g; the diameter ofthe particles generally ranges from 0.1 to 300 microns, preferably from1 to 120 microns.

This alumina provides a pore volume on the order of 0.10 to 0.50 cm³ /g,the pores having dimensions less than 50 nm.

In another preferred embodiment of the invention, the activated aluminaobtained by the rapid dehydration of Bayer hydrate (hydrargillite),which is an industrially readily available aluminum hydroxide that isrelatively inexpensive. Such an activated alumina is well known to thisart, and is described, in particular, in FR-A 1,108,011.

According to the process of this invention, activated alumina iscontacted with a complexing agent for the Al³⁺ cation prior to thematuring or aging operation, either in the state of a powder or in theform of agglomerates.

Exemplary complexing agents for the cation Al³⁺ include at least onecompound selected from among the acids comprising at least onecarboxylate group and at least two hydroxy and/or amine groups, orcomprising at least two carboxy groups and at least one hydroxy and/oramine group, and the salts thereof.

Representative such organic phosphoric acids have the following formulae(I) to (III): ##STR1## in which formulae (I) to (III), n and m areintegers ranging from 1 to 6; p is an integer ranging from 0 to 5; andR₁, R₂ and R₃, which may be identical or different, each represents ahydrogen atom or an alkyl, aryl, aralkyl, hydroxy or amino radical, or ahydroxyquinoline or derivative thereof corresponding to the followingformula (IV): ##STR2## in which formula (IV), R is a hydrogen atom, aC₁₋₂₀ hydrocarbon radical or a halogen atom.

Among the carboxylic acids useful as complexing agents for the Al³⁺cation, more particularly preferred are the C₂₋₁₅ and preferably C₂₋₁₀carboxylic acids.

Exemplary such acids include the following:

(i) oxalic acid;

(ii) hydroxypolycarboxylic acids, more particularly hydroxydi- orhydroxytricarboxylic acids, such as, for example, maleic acid, citricacid or tartronic acid;

(iii) (polyhydroxy)monocarboxylic acids, such as, for example,glucoheptonic and gluconic acids; and

(iv) poly(hydroxycarboxylic)acids, such as, for example, tartaric acid.

A mixture of these acids can be used equally as well. Preferably, citricor oxalic acid is used.

The salts of these acids are equally suitable according to thisinvention. The preferred salts are the salts of alkali metals such assodium salts, or ammonium salts.

Further as regards the complexing agents of formulae (I) to (III), anorganic, particularly aliphatic, substituted or unsubstituted organicphosphoric acid is preferably used. This may contain 1 to 15 andpreferably 1 to 10 carbon atoms.

Exemplary such compounds include methylene aminotriphosphonate,methylene ethylenediaminotetraphosphonate, methyltriethylenetetraaminohexaphosphonate, methylenetetraethylenepentaaminoheptaphosphonate, methylenepentaethylenehexaaminooctaphosphonate; the diphosphonates of methylene,1,1'-ethylene, 1,2-ethylene, 1,1'-propylene, 1,3-propylene, and1,6-hexamethylene; 2,4-dihydroxypentamethylene-2,4-diphosphonate;2,5-dihydroxyhexamethylene-2,5-diphosphonate;2,3-dihydroxybutylene-2,3-diphosphonate;1-hydroxybenzyl-1,1'-diphosphonate; 1-aminoethylene-1,1'-diphosphonate;hydroxymethylene diphosphonate; hydroxyethylene-1,1'-diphosphonate;1-hydroxypropylene-1,1'-diphosphonate;1-hydroxybutylene-1,1'-diphosphonate; and1-hydroxyhexamethylene-1,1'-diphosphonate.

For those complexing agents corresponding to formula (IV), preferred arethose compounds in which R is a hydrogen atom or straight or branchedchain saturated or unsaturated aliphatic radical preferably having from5 to 20 carbon atoms.

In a preferred embodiment, oxine (8-hydroxy-quinoline) is used, or an8-hydroxy-quinoline of formula (IV) in which every R is a hydrogen atom,except that at the 7-position in the ring R is an alkyl radical of theformula C_(m) H_(2m+1) with m ranging from 5 to 20.

The complexing agent such as defined above can be used in solid form or,preferably, in the form of an aqueous solution, the concentration ofwhich preferably ranging from 0.01 to 1 mole/liter.

The amount of complexing agent introduced with respect to the amount ofalumina is determined by the type of alumina that it represents,preferably ranging from 0.01% to 5% by weight of alumina and morepreferably from 0.05% to 0.5%.

Prior to agglomeration of the activated alumina, it can be advantageousto grind the activated alumina powder such that it is reduced toparticles having a mean diameter ranging from about 1 to 20 microns.

Crushing, whether in the moist or dry state, is typically carried out.

It is possible to effect such crushing by a jet of air, but most oftenthe crushing is carried out in a conventional manner in a ball grinder,dry or with the addition of water.

A first embodiment of the process of the invention entails introducingthe complexing agent over the course of the grinding operation, inparticular by spraying an aqueous solution containing the complexingagent onto the activated alumina powder.

This operation of crushing is optional and in its absence it isnecessary to add the complexing agent in the following step.

Another embodiment of the invention entails adding the complexing agentover the course of the agglomeration operation.

The agglomeration of activated alumina is carried out according tomethods per se known to this art such as, for example, by pelleting,extrusion, shaping into spheres in a rotary pelleting machine, and thelike.

This agglomeration can be carried out on activated alumina such as thatproduced by dehydration and optional subsequent treatments, or on anactivated alumina crushed to one or more predetermined granulometries.

This agglomeration can be effected in a manner well known to this art,by adding porogenic agents to the mixture to be agglomerated. Porogenicagents that can be used include, in particular, wood flour, wood carbon,cellulose, starches, naphthalene, and, in general, any compound thatwill be eliminated by calcination.

A preferred embodiment of the invention entails spraying an aqueoussolution containing the complexing agent onto an activated aluminapowder in a state of agitation, for example rotating in a pelletingmachine.

In another embodiment of the process of the invention, the complexingagent can be added to the activated alumina agglomerates prior to thematuring operation.

The addition of the complexing agent can be carried out, in particular,by spraying an aqueous solution containing the complexing agent onto thesaid agglomerates.

Whether the complexing agent is added at the time of maturing or in apreceding step, the activated alumina agglomerates are subjected to thematuring operation, over the course of which the alumina is rehydratedand the boehmite crystalline phase is developed.

The operation of maturing, or aging, is carried out at a temperatureranging from 80° to 120° C., preferably at about 100° C.

The duration of the maturing is variable and preferably ranges from 1 to24 hours, more preferably from 2 to 6 hours.

A preferred embodiment of the maturing step entails quick injection ofwater vapor onto the alumina agglomerates.

In accordance with the process of the invention, the activated aluminaagglomerates obtained after maturing are subjected to a thermaltreatment in order to stabilize the specific surface of the alumina andpromote the adsorbent properties of the agglomerates.

The thermal treatment is carried out at a temperature ranging from about100° to 500° C., for a period of time sufficient to remove the free andstructural water.

Over the course of this step, the porogenous agent can be decomposed.

It is possible to carry out the thermal treatment in a single operation,for example in a rotating industrial furnace where the temperature isgraded in the temperature range illustrated, depending on the positionof the agglomerates with respect to the heat source, which is preferablya gas burner.

In this event, the duration of the thermal treatment preferably rangesfrom 2 to 12 hours.

The thermal treatment can be effected equally well in two conventionalsteps: drying, then calcination.

The activated alumina agglomerates can then be dried at a temperaturegenerally ranging from about 100° to 250° C., for a period of time mosttypically ranging from 2 to 24 hours.

They are subsequently subjected to a calcination operation at atemperature ranging from 250° to 500° C. over a period of time rangingfrom, for example, 1 to 8 hours.

After thermal treatment, the activated alumina agglomerates obtainedessentially consist of alumina. Their ignition weight loss is on theorder to 2% to 6%.

As indicated above, the subject alumina agglomerates are preferably inthe form of small spheres having a mean diameter ranging from 1 to 10mm.

By "mean diameter" is intended a diameter such that 50% by weight of thespheres have a diameter greater or less than the mean diameter. The meandiameter is determined by sifting using different sieves having openingsof defined mesh.

The textural properties of the activated alumina agglomerates of theinvention are set forth below.

Their BET specific surface area generally ranges from 250 to 400 m² /g,preferably from 290 to 350 m² /g.

The specific surface area expressed is a BET specific surface, i.e.,determined by nitrogen adsorption conforming to ASTM standard D 3663-78,established by the Brunauer-Emmet-Teller method (Journal of the AmericanChemical Society, 60, 309 (1938)).

Their total pore volume TPV most typically ranges from 0.40 cc/g to 0.60cc/g.

The total pore volume is determined by measuring the specific gravities.

The total pore volume TPV is given by the following formula: ##EQU1##where Dg=density of the particle and Da=absolute density.

The particle and absolute densities Dg and Da, respectively, aremeasured by the pycnometric method using mercury and helium; then theTPV is calculated from the above formula.

The activated alumina agglomerates according to the invention have atotal packing density TPD ranging from about 500 to 1,00 g/cc.

To determine the TPD, a given weight of agglomerates is introduced intoa graduated cylinder to provide a given volume. The cylinder is thenvibrated until packing ceases and a constant volume is obtained. Theweight of the agglomerates that occupy a unit volume is calculated.

Insofar as the mechanical properties of the agglomerates is concerned,they possess a very good resistance to crushing as defined above.

The measure of crushing resistance particle by particle CR is carriedout in a Lhomargy DY.20 B crushing apparatus, the jaws of which areclosed at a speed of 0.5 mm/minute.

10 spheres are removed at random from the granulometric fraction 3.15 to4 mm. They are placed successively between the hammer and the anvil ofthe apparatus using Brucelles pincers.

The final result (expressed in daN) is given as the mean of the crushingstrength of the 10 spheres.

The activated alumina agglomerates of the invention display an equallygood resistance to attrition, greater than about 98%.

The attrition resistance is measured as the percentage of product notconsumed by friction according to a method described below, whichentails subjecting a predetermined weight of agglomerates to intenseagitation and measuring the amount of agglomerates remaining that havenot been reduced to dust.

Before carrying out this test, the agglomerates are conditioned by athermal treatment for two hours in a furnace, the temperature of whichis regulated at 300° C.

After cooling in a desiccator, the product is sieved to its lowestnominal dimension using an Afnor sieve. For example, if the spheres havea diameter of 1.6 to 2.5 mm, the sifting is to 1.6 mm. A precise amountof the sifted product (about 10 g) is weighed out.

This amount is placed in a 65 cc stainless steel grinder vessel in ashaker marketed by Prolabo under the trademark Dangoumau microgrinder.The microgrinder is operated for exactly 5 minutes.

When the agitation is completed, the product recovered is sifted using asieve corresponding to three quarters of the smallest dimension of thestarting material.

The sifted product is placed in a furnace for two hours at 300° C.

After cooling in a desiccator, the agglomerates that do not pass throughthe sieve are weighed.

The attrition resistance AR is given by the following formula, where W₁is the weight of the sample and W₂ is the weight of the agglomeratesremaining on the sieve after testing: ##EQU2##

As well as having very good mechanical properties, the activated aluminaagglomerates according to the invention have a good adsorption capacityEO.6, as high as 25%.

The test for determining the adsorption capacity entails measuring theamount of water fixed by the activated alumina in an atmosphere when thevapor pressure of water is equal to 60% of saturation.

This pressure is attained above a saturated aqueous solution of sodiumbromide maintained at 15° C.

To eliminate the water that the sample has adsorbed during its exposureto the above atmosphere, the sample is regenerated for two hours at 300°C. before analysis. An exact amount, in the vicinity of 2 g, of thealumina agglomerates is weighed in a weighing bottle. The weighingbottle is placed, open, in a vacuum desiccator containing sodiumbromide. After the pressure in the desiccator has been reduced to 15 mmof mercury, it is placed for twenty-four hours in a thermostattedenclosure at 15° C. The increase in weight after this treatment ismeasured.

The adsorption capacity EO.6 is given by the following formula, where Wis the weight of the sample and W₁ is its increase in weight. ##EQU3##

Without wishing to be bound to any particular theory, it is believedthat the desirable properties of the activated alumina agglomerates ofthe invention can be correlated to the structure of the agglomeratesobtained before maturing (aging) and that the presence of boehmite atthe periphery or face surface of the agglomerates imparts to them anexcellent resistance to crushing.

As indicated above, a preferred embodiment for carrying out theinvention comprises preparing activated alumina agglomerates by theprocedure described above and selecting citric acid as the complexingagent.

It is most significant that the intermediate alumina agglomerates, i.e.,the agglomerates obtained following maturing, like the finalagglomerates obtained after calcining, all display a wholly originalsurface morphology.

The intermediate agglomerates or small spheres of alumina comprise acore essentially consisting of poorly crystallized (chi) alumina andamorphous alumina, of boehmite and of bayerite. In said agglomerates,the amount of boehmite is preferably greater than 5% and more preferablyranges from 8% to 20%.

The small spheres of aged alumina have the appearance, on the surfacesthereof, of excrescences constituted by cylindrical balls whosedimensions are very variable. This external surface of said spheres ischaracteristic.

FIG. 1 is an electron photomicrograph (enlargement 10,000) showing thatthe surface morphology of the small spheres of matured alumina producedaccording to the invention, using citric acid as the complexing agent atthe time of crushing.

FIG. 2 is a like electron photomicrograph showing, by way of comparison,the morphology of small spheres of matured alumina produced by the sameprocedure, but without using a complexing agent.

The presence of rods having a length ranging from 0.5 to 2 microns isseen only in FIG. 1.

The final alumina agglomerates that are therefore obtained after thermaltreatment present a surface morphology resembling that of theintermediate alumina agglomerates.

Comparison of FIGS. 1 and 2 indicates, significantly, the influence ofthe presence of the complexing agent on the morphology of the smallspheres of alumina produced.

By virtue of their properties, the activated alumina agglomerates of theinvention are very well adopted for use as adsorbents, in particular forthe purification of gases by adsorption of CO₂, CO, N₂, NH₃,hydrocarbons, etc.

They can also be used as supports in chromatography and in the field ofcatalysis.

In order to further illustrate the present invention and the advantagesthereof, the following specific examples are given, it being understoodthat same are intended only as illustrate and in nowise limitative.

EXAMPLE 1

The starting material was an activated alumina produced by dehydrationof hydrargillate and having the following characteristics:

(i) alumina having a PAF of 5%, constituted by chi and amorphous alumina(40%);

(ii) Na₂ O content 0.3%;

(iii) BET 300 m² /g;

(iv) Total pore volume TPV 0.30 cc/g;

(v) Total packing density TPD 0.30 cc/g.

75.4 kg of this activated alumina were crushed in a ball-mill to obtaina powder, the mean diameter of whose particles was 10 microns.

During the crushing operation, which was carried out for 45 minutes, anaqueous solution of citric acid prepared by dissolving 68 g of citricacid in 4.75 liters of water, corresponding to a ratio citric acid/Al₂O₃ of 0.1%, was sprayed onto the powder.

It was then converted into crushed powder in a granulator. Thetemperature of the granulation was about 50° C. To facilitate thisoperation, water was added.

The agglomerates or small spheres of alumina produced had a meandiameter of 3.35 mm, with a loss on heating of about 35%.

Upon removal from the granulator, the particles were stored in a jar andthen subjected to a step of maturing by exposure to water vapor at 100°C. over four hours.

X-ray diffraction analysis of the matured small spheres revealed 9% ofbayerite and 12% of boehmite.

FIG. 1 illustrates the surface morphology of the matured spheresproduced.

After the maturing step, the spheres were dried for 2 hours, 30 minutes,at 220° C. and calcined for 2 hours, 30 minutes, at 450° C.

The final product spheres had the following characteristics:

(a) BET specific surface, 310 m² /g;

(b) Total pore volume TPV, 0.41 cc/g;

(c) Total packing density TPD, 0.830 g/cc;

(d) Crushing resistance CR, 20 daN;

(e) Adsorption capacity EO.6, 21.5%;

(f) Attrition resistance, AR 99%.

EXAMPLE 2

The procedure of Example 1 was repeated, except that over the course ofthe grinding an aqueous solution of oxalic acid was sprayed onto thepowder. It was prepared by dissolving 175 g of oxalic acid in 4.75liters of water, which corresponds to a ratio oxalic acid/Al₂ O₃ of0.25%.

After granulating the powder, small spheres were obtained which weresubjected to maturing.

The analysis by X-ray diffraction of the matured spheres showed 4% ofbayerite and 15% of boehmite.

The spheres obtained after drying and calcination had the followingcharacteristics:

(a) BET specific surface, 325 m² /g;

(b) Total pore volume TPV, 0.45 cc/g;

(c) Total packing density TPD, 0.780 g/cc;

(d) Crushing resistance CR, 15 daN;

(e) Adsorptive capacity EO.6, 22%.

COMPARATIVE EXAMPLE A

In this test, the complexing agent was not used.

The procedure of Example 1 was repeated, except that 76 kg of activatedalumina were crushed and 4.75 liters of water were sprayed onto thepowder over the course of the crushing.

After granulating the powder, spheres were obtained that were submittedto maturing.

X-ray analysis of the matured spheres revealed 26% of bayerite and 2% ofboehmite.

FIG. 2 illustrates the surface morphology of the matured spheresproduced.

The spheres obtained after drying and calcination had the followingcharacteristics:

(a) BET specific surface, 330 m² /g;

(b) Total pore volume TPV, 0.43 cc/g;

(c) Total packing density TPD, 0.800 g/cc;

(d) Crushing resistance CR, 11 daN;

(e) Adsorption capacity EO.6, 23%;

(f) Attrition resistance AR, 98.6%.

By comparing Examples 1 and 2 and Comparative Example A, it will be seenthat the presence of a complexing agent added over the course of thecrushing retards the appearance of bayerite and favors that of boehmiteunder conditions that normally provide a predominance on bayerite.

There results an increase in the mechanical resistance EGG of the smallspheres of activated alumina.

It is also seen that the good adsorption capacity is retained.

In order to compare the crushing resistance of the small alumina spheresobtained by the process of the invention (Example 1) using citric acidas the complexing agent with the small spheres obtained in the absenceof a complexing agent (Comparative test A), the crushing resistance ofthe spheres was determined as a function of their diameter.

FIG. 3 is a graph showing two lines (A) and (B) which represent thevariation in the crushing resistance CR in daN as a function of thesquare of the diameter of the sphere in mm².

The variation in CR as a function of d² corresponds, respectively:

in the case of the spheres of Example 1 of the invention, to theequation y=1.3x (line A),

in the case of the spheres of Comparative Example A, to the equationy=0.7 x (line B).

A comparison of the slopes of the lines indicates the net increase inthe resistance to crushing of the alumina spheres of the invention.

While the invention has been described in terms of various preferredembodiments, the skilled artisan will appreciate that variousmodifications, substitutions, omissions, and changes may be made withoutdeparting from the spirit thereof. Accordingly, it is intended that thescope of the present invention be limited solely by the scope of thefollowing claims, including equivalents thereof.

What is claimed is:
 1. Activated alumina agglomerates having a crushresistance of at least 15 daN, measured on that fraction thereof havinga mean diameter ranging from 3 to 4 mm, and an adsorptive capacitygreater than 18% by weight, wherein the adsorptive capacity is expressedas a percentage by weight of water adsorbed and the activated aluminaagglomerates having a surface morphology thereof comprising microrods.2. The activated alumina agglomerates as defined by claim 1, having acrush resistance not greater than 25 daN.
 3. The activated aluminaagglomerates as defined by claim 2, having a crush resistance rangingfrom 15 to 20 daN.
 4. The activated alumina agglomerates as defined byclaim 1, having a crush resistance greater than 22 daN, measured on thatfraction thereof having a mean diameter ranging from 6 to 7 mm.
 5. Theactivated alumina agglomerates as defined by claim 4, having a crushresistance greater than 25 daN.
 6. The activated alumina agglomerates asdefined by claim 5, having a crush resistance ranging from 30 to 50 daN.7. The activated alumina agglomerates as defined by claim 1, wherein thevariation in their crush resistance as a function of the square of thediameter thereof is according to the equation y=1.3 x, wherein y is thecrush resistance and x is the square of the diameter.
 8. The activatedalumina agglomerates as defined by claim 1, having an absorptivecapacity ranging from 20% to 24%.
 9. The activated alumina agglomeratesas defined by claim 1, comprising alumina spheres having a mean diameterranging from 1 to 10 mm.
 10. The activated alumina agglomerates asdefined by claim 1, having a BET specific surface area ranging from 250to 400 m² /g.
 11. The activated alumina agglomerates as defined by claim10, having a BET specific surface area ranging from 290 to 350 m² /g.12. The activated alumina agglomerates as defined by claim 1, having atotal pore volume ranging from 0.40 to 0.60 cc/g.
 13. The activatedalumina agglomerates as defined by claim 1, having a total packingdensity ranging from 500 to 1,000 g/cc.
 14. The activated aluminaagglomerates as defined by claim 1, having a resistance to attrition ofgreater than 98%.
 15. A process for the preparation of the activatedalumina agglomerates as defined by claim 1, comprising agglomeratingactivated alumina particulates, maturing the resulting agglomerates inthe presence of a complexing agent for the cation Al³⁺ and thenthermally treating the matured agglomerates at a temperature rangingfrom about 100° to 500° C.
 16. The process as defined by claim 15,comprising maturing under atmospheric pressure.
 17. The process asdefined by claim 15, said starting material activated aluminaparticulates having been produced by rapid dehydration of hydrargillite.18. The process as defined by claim 15, said complexing agent comprisingan at least monocarboxylic acid having at least two hydroxy and/or aminefunctional groups, or salt thereof.
 19. The process as defined by claim15, said complexing agent comprising an at least dicarboxylic acidhaving at least one hydroxy and/or amine functional group, salt thereof.20. The process as defined by claim 15, said complexing agent comprisinga phosphorus acid having one of the following formulae (I) to (III):##STR3## in which formulae (I) to (III), n and m are integers rangingfrom 1 to 6; p is an integer ranging from 0 to 5; and R₁, R₂ and R₃,which may be identical or different, each represents a hydrogen atom oran alkyl, aryl, aralkyl, hydroxy or amino radical.
 21. The process asdefined by claim 15, said complexing agent comprising a hydroxyquinolineof formula (IV), or derivative thereof: ##STR4## in which formula (IV),each R is a hydrogen atom, a C₁₋₂₀ hydrocarbon radical or a halogenatom.
 22. The process as defined by claim 15, said complexing agentcomprising a C₂ -C₁₅ carboxylic acid.
 23. The process as defined byclaim 15, said complexing agent comprising oxalic acid.
 24. The processas defined by claim 15, said complexing agent comprising ahydroxypolycarboxylic, (polyhydroxy)monocarboxylic orpoly(hydroxycarboxylic) acid.
 25. The process as defined by claim 15,said complexing agent comprising malic, citric, tartonic, glucoheptonic,gluconic or tartaric acid.
 26. The process as defined by claim 15, saidcomplexing agent comprising methylene aminotriphosphonate, methyleneethylenediaminotetraphosphonate, methyltriethylenetetraaminohexaphosphonate, methylenetetraethylenepentaaminoheptaphosphonate, methylenepentaethylenehexaaminooctaphosphonate; a diphosphonate of methylene,1,1'-ethylene, 1,2-ethylene, 1,1'-propylene, 1,3-propylene, and1,6-hexamethylene; 2,4-dihydroxypentamethylene-2,4-diphosphonate;2,5-dihydroxyhexa-methylene-2,5-diphosphonate;2,3-dihydroxybutylene-2,3-di-phosphonate;1-hydroxybenzyl-1,1'-diphosphonate; 1-amino-ethylene-1,1'-diphosphonate;hydroxymethylene diphosphonate; hydroxyethylene-1,1'-diphosphonate;1-hydroxypropylene-1,1'-diphosphonate;1-hydroxybutylene-1,1'-diphosphonate; or1-hydroxyhexamethylene-1,1'-diphosphonate.
 27. The process as defined byclaim 15, wherein the amount of said complexing agent ranges from 0.01%to 5% by weight of said activated alumina particulates.
 28. The processas defined by claim 15, comprising crushing said activated aluminaparticulates in the presence of said complexing agent, upstream of theagglomeration thereof.
 29. The process as defined by claim 15,comprising agglomerating said activated alumina particulates in thepresence of said complexing agent.
 30. The process as defined by claim15, comprising adding said complexing agent to said agglomerates,upstream of the maturing thereof.
 31. The process as defined by claim15, comprising maturing at a temperature ranging from 80° to 120° C. 32.The process as defined by claim 15, comprising drying the maturedagglomerates.
 33. The process as defined by claim 15, further comprisingcalcining the resulting agglomerates.
 34. Agglomerates of activatedalumina comprising a core matrix essentially consisting of poorlycrystalline or amorphous alumina and a sheath thereon which comprisesmicrorod excrescences., the agglomerates comprising greater than 5% ofboehmite and at least 30% of bayerite.
 35. The agglomerates as definedby claim 34, comprising from 8% to 20% of boehmite.